Sound reproducing system



June 9, 1942. H. 1., FORESTER I 8 SOUND REPRODUCING SYSTEM Filed May 9, 1941 IP II INVENT OR f/ARRY L F095 75/? [BY km:

ATTORNEY Patented June 9, 19.42

- IUNITED- STATES PATENT OFFICE 2,285,76'9' SOUND REPRODUCING SYSTEM Harry L. Forster, Bnflalo, N. ,Y., assignor to Colonial Radio Corporation, Buifalo, N. 1.

Application May 9, 1941, Serial No. 392,094

8 Claims.

This invention relates to electrical sound reproducingsystems involving amplifiers and loud speakers and more particularly to such a system particularly, adapted to be employed in radio 1'6? ceivers, electric phonographs, public address systems and the like,

It is well known that most loud speakers of the so-called direct acting type, embodying no attention or adjustment on the part of the ophorn, have an undesired resonance peak. This peak is ordinarily located within the range of audio frequencies desired to be reproduced and in most cases falls near the low frequency end of the band. For example, a typical baflle loaded cone loud speaker such as is ordinarily employed in home radio receivers has a pronounced resonance peak in the neighborhood of 200 cycles. Such a resonance peak is undesirable under most circumstances because it tends to unduly increase the sound output in the neighborhood of the resonant frequency, thereby upsetting the desired tonal balance over the audio range.

Such a resonance peak is sometimes relied upon to increase the low frequency response of the speaker and particularly where the design of the speaker is such that if such peak were not present the low frequency response would be less than desired, but even in such cases the increase frequencies.

It is a further object of this invention to pro- 'erator.

' Still other objects and advantages of my invention will be apparent from the specification. In this application I have particularly pointed out and distinctly claimed the part, improvement or combination which I claim as my invention or discovery, and I have explained the principles thereof and the best mode in which I have contemplated applying those principles so as to and vide such a sound reproducing system in which the amplification may be adjusted to compensate for the low frequency resonance peak usually present in the speaker, whereby the output of the system may be made more nearly uniform in spite of the resonance effect.

It is still a further obiectof my invention to provide an audio amplifying system which will have positive regeneration of a portion of the audio spectrum, usua1ly the lower portion, zero regeneration another portion, usually higher, and negative regeneration over another portion, usually still higher.

It is still a further object of this invention to provide a system of the class described in which zero regeneration is supplied at the resonance peak of the loud speaker, positive regeneration for frequencies below the resonance peak and distinguish my invention from other inventions. In the drawing: vFig. 1 is a circuit diagram of a system in accordance with my invention;

Fig. 2 is an'equivalent network diagram useful in understanding the operation of my invention,

Fig. 3 is a vector diagram illustrating the voltage and current relationships for one particular frequency, a

Referring now more particularly to Ffig. 1, in practicing my inventionl prefer to employ an audio frequency amplifier of the so-called resistance coupled type. This amplifier may employ the first amplifier tube I having a cathode la, a grid or control electrode lb and an anode or plate lc. Audio frequency voltages may be impressed between the cathode la and control electrode lb from any suitable output such as the detector of a radio receiver, a phonograph pick-up, a microphone or the like. The anode to may be connected through a resistance 2 to a suitable source of potential indicated as +B.

Coupling condenser C; may be connected to the top of resistance 2 and to control electrode or grid 31 of the second amplifier tube 3 which may have cathode 3a and plate or anode 3c. The control electrode 3b may be connected through resistance R1 to ground. The primary winding 4a of a suitable output transformer may be connected on the one end to the anode 3c and on the other end to a suitable source of potential indicated as +B. Associated with the coil la may be the core to and the secondary winding lb one end of which may be connected to ground. The secondary winding may feed the voice coil in of loud speaker 5. v

For the purposes of this discussion, it maybe assumed that the loud speaker 5 is a conventional loud speaker of the so-called dynamic type in which the voice coil moves axially in an annular magnetic field excited by either an electronet or a permanent magnet and drives a cone diaphragm. v

The ungrounded' terminal of secondary winding 4b may be connected through condenser C: and resistance R; in series to the grid lb of the tube I. In making this connection, care should be taken to obtain proper phasing. If reverse connections are made, pronounced oscillations are likely to occur. The grid lb may be connected to ground through resistance R4 and the resistances R3 and R4 together may be designated By proper choice of the values of resistances R1 and R2 and condensers C1 and C2, the circuit may be made to have positive regeneration at the low frequency end of the audio band, zero re generation at a frequency above the frequencies of. positive regeneration, and negative regeneration for all frequencies above the frequency of zero regeneration. By proper selection of these values the cross-over frequency (the frequency of zero regeneration) may be placed at any point within the audio resistance band, but I prefer to place it at or near the undesired low frequency resonance peak ofthe speaker.

I have found thatbest results are obtained for a typical small receiver when C1 has a value in the neighborhood of .005 microfarad, R1 250,000 ohms, C2 750 m. m, f. and R2 2 megohms. With such values, and using a speaker having an undesired resonance peak at 200 cycles, I am able to supply 8 decibels of positive regeneration at 90 cycles, 10 decibels of negative regeneration at 400 cycles, and zero regeneration at 200 cycles,

thus giving a bass peak at 90 cycles, 18 decibels vabove the response at 400 cycles.

are similarly related to the impedance of the grid circuit of the tube I which usually will not be less than that of the tube 3 and may be considerably greater. V

B: may be made to fall within the range from .3. megohm to 10 megohms and correspondingly C: may vary from .009 microfarad to micromicrofarads. In any case the ideal relation between C1 and R1 and C: and R: is that 1 V 270" were I is the'frequency'for which zero regeneration is desired. A certain latitude is permissible l q 276- at a frequency 1. It can be shown that there are definite mathematical relationshinps be-.

tween the voltages E and E2 acroa terminals I, 2 and I, 4 respectively, E1, the voltage across condenser C, and I, the current in the network.

Considering terminals 3 and 4 to be open and the voltage E applied between terminals I and 2, it will be seen that the current I in the circuit flows through terminal I, condenser C, resistance R and terminal 2 in that order during one-half of the cycle and in the reverse direction during the other half. Since a common current is flow- 7 ing through C and R, and since the voltage acrou a condenser always lags the current through it' v by approximately 90, and the voltage across a in that the reactance of the condenser may be made as small as 0.4 R or as large as 1.6 R,'but ordinarily it will not be desirable to exceed these limits.

It is my belief that the explanation for the operation of the-circuit is to be found in the variation in'phase relations which occur in the two resistance capacity networks, the first being Ca-R: coupling the output of the amplifier as a whole backto the input, and the second being (Ii-R1 coupling the output of the first tube to the input of the second tube. These two netresistance is always in phase with the current through it, it follows that the voltagesacross condenser C and' resistance R are 90 out of phase, the voltage across condenser C lagging the voltage across resistance R. Since by assumption the reactance of condenser C is equal to the resistance R at the frequency 1, the voltage drops across them are equal in magnitude.

The voltage relationships for any particular frequency, for example frequency f, at which the reactance of condenser C eq may be plotted and this has been done in Fig. 3. E is the input voltage. It is clear that the load current leads the impressed voltage by 45 so I may be laid off 45.ahead of E. Since E: is in phase with I, E: is laid off on the same vector as I and since E1 lags I by 90, it may be laid oi! as shown.

Having established the fact that E: leads E by 45 when the reactance of condenser C equals resistance R for a frequency f, we now assume another frequency, 2!. For the frequency 2! the voltage drop across C is one-half of that across R, since the reactance of condenser C is equal to one-half R at the frequency 2!, and the phase angle between the current and voltage is I tan 4 or about 27, and En leads E by 27".

At a third frequency An leby M ltude of M I quancy Eglneads Damn R 45 ls 2R M I Since we have in the present instance two such networks in series, in the circuit, the amount of works are of the same pattern and if proper shift will be double. that shown in the the resistance R,

Arranging these in the table for any particular frequency and again tabulating we have the following:

Anleby Frew ch 1 1 Donut b2 90 [2 128 scribed and produces the results above noted, as

proved by repeated tests.

Itwillbenotedthatthe divisionoflisintotwo components It: and Reforms a voltage divider limiting the amount of feedback. Variation of the relative values of Rs and R4 may control the amount of feedback, but so long as R; is not charged in relation to C: the crossover frequency so 8. In an audio frequency amplifier, in comwill remain substantially constant.

While I have shown and described certain preferred embodiments of my invention,-it will be understood that modifications and changes may bernade without departing from the spirit scope of my invention.

I claim:

i. In an audio frequency amphfier, in combination, a first amplifier tube, a second amplifier tube, a non-resonant resistance-capacity network coupling the plate circuit offthe first tube to the grid circuit of the second tube. and a resistance capacity network coupling the output circuit of said second tube to the grid circuit of the first tube, the values of resistance and ca- 7 pacity in said networks beingsuch that the 'reactance of the capacity in each network is equal a to the resistance in said network at a frequency 1 within the audible range.

2. In an audio frequency amplifier, .in combination, a first amplifier tube, a second amplifier tube, a non-resonant resistance-capacity network coupling the plate circuit of the first tube y to the grid circuit of the second tube,'and a resistance capacitynetwork coupling the output circuit of said second tube to the grid circuit of E the first tube, the values of resistance'and capacity in said networks being such that the reactanoe of the capacity in each network is equal to the resistance in said network at a frequency within theaudible range, and adjacent the low frequency end thereof.

8. In an audio frequency amplifier, in combination, a first amplifier tube, a second amplifier tube, means for coupling said tubes in cascade, a non-resonant reaction coupling from the out-.

put circuit of said second amplifier tube to the input circuit of the first amplifier tube, the constants of said' couplings being so chosen that the phase relations of energy fed back are such as to provide positive regeneration ever part of the audible range, and-negative regeneration at 4. The combinationwith a loud speaker having undesired resonance near the low-frequency end frequencies above said part of the audible range.

said second tube to the input circuit of the first tube, an audio circuit coupling the output circuit of said second tube and said loud speaker, the values ofsaid resistance-capacity network 10 being so chosen that said amplifier has positive regeneration below the loud-speaker undesired resonant frequency, and negative above said frequency.

5. The combination with a loud speaker havlli ing undesired resonance near the low-frequency end of the audio band, of an amplifier for feeding said speaker comprising a pair of amplifier tubes in cascade. a non-resonant resistance-capacity network coupling the plate circuit of the first 2o tube to the grid circuit of the second tube, a resistance-capacity network coupling the output circuit of said second tube to the input circuit of the first tube. an audio circuit coupling the output circuit of said second tube and said loud g5 speaker, the values of'the resistance and capacity in said networks being so chosen that the reactance of the capacity in each network is equal to the resistance of the network at the .undesired resonant frequency of the loud speaker bination, a first amplifier tube, a d amphfier tube, a non-resonant resistance-capacity network coupling the plate circuit ofthe first tube to the grid-circuit of the second tube, and a 35 resistance-capacity network coupling the output circuit of said second tube to the grid circuit of said second tube to the grid, circuit of the first tube, the values of resistance and capacity in saidnltworks being so related that the angle of phase to shift introduced by each network is substantially equal- I. In an audio frequency amplifier, in combination, a first amplifier tube, a second amplifier tube, a resistance-capacity network coupling the plate circuit of the'first tube to the grid circuit of the second tube, and a non-resonant resistance-capacity network coupling the output circuit of the second tube to the input circuit of the first tube,.the capacity of each network beingrelatedtotheresistancether'eofintherelation the crossover frequency.

8. The combination with a loud speaker havpacity network coupling the plate circuit of the first tube tothe grid circuit of the second tube, a

resistance-capacity network couplingthe output circuit of said second tube to the input circuit of the first tube, an audio circuit coupling the output circuit of said. second tube-and said loud speaker, the capacity-of each network being related to the resistance thereof in the relation where f is the undesired resonance frequency of said loud speaker.

I HARRY L. I'ORSTER.

where a a the resistance of the network and 1 is 

